This invention relates to a device for accelerating solid particles entrained in a carrier gas through a duct. More particularly, this invention relates to a new and improved acceleration nozzle for solid particles entrained in a carrier gas. Such acceleration nozzles are typically used for introducing carboniferous powdered materials into a steel bath.
The ratio of scrap iron or other cooling additives advantageously incorporated into a metal bath during the refinement process depends chiefly on the composition of the smelting material, the temperature of the charge and the thermodynamic development of the refining process. It will be appreciated that to reduce the cost or price of steel, it is important to increase the ratios of presently used additives by some 400 kg of scrap iron per ton of smelting. One of the known methods for increasing the amount of scrap during refining consists of increasing the level of postcombustion CO being given off from the bath, while insuring that the bath absorbs a maximum amount of the heat liberated therefrom. Another known method consists of heating the metallic bath by utilizing supplementary sources of energy. Techniques of addition of gas and combustible liquid are used with varying success. Also, techniques of addition of combustible material in the form of granules of carbureted matter have been developed. The incorporation of solid material in the bath may be accomplished from below (through nozzles or permeable elements positioned in the bottom of the converter), or from above the converter together with gaseous materials. Thus, in this last case (solid materials entrained in a carrier gas), in order to have a suitable absorption of the carbureted material by the bath, the solid carbureted material must have not only well established concentrations of oxygen and carbon, but in addition, the carbureted material must have sufficient kinetic energy and concentration at the outlet of the nozzle so as to penetrate the bath. The high kinetic energy is also required to avoid premature combustion of the carbureted material above the bath.
In patent application EP No .84630036 corresponding to U.S. patent application Ser. No. 587,540 now U.S. Pat. No. 4,603,810,assigned to the assignee hereof, and incorporated herein by reference, a device is described for the acceleration of solid particles suspended or entrained in a carrier gas. The device comprises a source of gas under pressure, means of obtaining the proper mixture of gas and solid particles together with delivery conduits for the gas/solid particle mixture opening into a nozzle. An important and novel feature of the nozzle disclosed in U.S. Pat. No. 4,603,810 is that at least a portion thereof has a cross-section which varies in a specific manner. This varying nozzle cross-section precludes the speed of the gas from increasing abruptly along the last few meters of the nozzle (as this relatively high gas speed can no longer be transmitted to the solid particles). By choosing nozzles or ducts that flare out, i.e., diverge, along the last meters before the mouth or exit opening, particle speeds of some 190 m/s have been obtainable at the mouth; the speed of the gas at that point being slightly less than the speed of sound.
Although the device described in U.S. Pat. No. 4,603,810 provides excellent results from the standpoint of particle speed and is therefore well suited for its intended purposes, it does suffer from several deficiencies and drawbacks. For example, it has been found that the depth of penetration of the solid particles into the bath is poor. Theoretical calculations show that the depth of penetration L of a jet of particles into a bath of liquid equals (without the presence of oxygen jets and for slight angles of divergence A and high concentrations of particles): ##EQU1## where:
Q.sub.c =particle discharge (kg/min);
L.sub.o =height of the nozzle above the bath (m);
v.sub.c =particle speed (m/s);
.rho.ac=steel density (kg/m.sup.3);
A=divergence angle of jet (degrees);
Blank tests in the atmosphere have shown that the angle A is contained between 4.degree. and 7.degree. , from which, with the aid of equation (1), a penetration depth L of from 15 to 50 cm. (with Q.sub.c =300 kg/min, v.sub.c =150 m/s, L.sub.o =1.5 m) may be calculated.
However, under actual conditions, the real penetrations of solid particles are far from the ideal conditions that led to equation (1). Under real conditions, account must be taken of the fact that at the time of the recarburization:
(a) The vertical blast nozzle of the entrained mixture of gas and solid material is surrounded by several blast pipes of primary oxygen which induce an increase in the angle of divergence A of the gas/solid particle jet. The effect of the exhaust of the oxygen jets actually gives rise to a depressurization of the central area which is surrounded thereby, and in which the gas/solid particle jet moves. This jet, the static pressure of which at the mouth is of 1 bar, consequently undergoes an abrupt expansion causing a radial displacement of the particles and as a consequence thereof, a diminution of their concentration.
(b) Also, as the carrier gas travels through the liquid bath, the gas slows thereby creating a counter-current which enlarges the area of impact on the bath. As a result, the carrier gas does not enter into the steel bath. Instead, it is strongly decelerated at the surface of the bath, which results in a diminution of the dynamic pressure and a corresponding increase of the static pressure. A pressure gradient is established in the area contained between the oxygen jets and the central jet which is a generator of counter-currents absorbed progressively by the jets. These counter-currents reinforce the shearing action between the central jet and the atmosphere that surrounds it.
(c) Finally, the difference between the speed of the carrier gas (Approx. 320 m/s) and the particles (approx. 180 m/s) at the outlet of the nozzle creates additional micro-turbulences inside the jet.
It follows that the angle of divergence A of the jet of particles in the crucible must be distinctly greater than that observed in the blank tests. If A becomes greater than the limit value calculated below, ##EQU2## where:
.DELTA.t="opening time" of the bath; and
d.sub.o =outlet diameter of the nozzle.
shows that the actual or real depth of penetration L is no more than several centimeters.